1. Field of the Invention
The present invention relates to a sealed motor, and more particularly, to the motor of which the housing is contrived for easy and reliable positioning of rotors and a stator core.
2. Description of the Related Art
Hitherto, motors which are installed in vehicles are demanded to be small, precise, and durable in harsh environments. Accordingly, a small and precise stepping motor with a sealed-structure is employed for adverse environments such as a dusty or a moist environment. This type of stepping motor typically has a structure in which a stator core is disposed outside rotors which are attached to a rotating shaft, and the front portion and the back portion of the rotating shaft are held by bearings which are attached to bushes disposed at the front and the back of the stator core, respectively. In this conventional structure, the bushes have been generally made of aluminum alloy by die sting or cutting. In the structure, rotors are attached to the circumference of a rotating shaft. Outside the rotors, the stator core, which is wound with a stator coil, is spaced at a very small predetermined interval.
Each of the bushes is joined with the front portion and the back portion of the stator core. The bushes are made of aluminum alloy by die casting or by cutting with high precision such that the outer end portion of each of the bushes firmly contacts the entire circumferential surface of the stator core, and the bushes sandwich the stator core with bolts. Each of the bushes has an inner circumferential portion inside of the outer circumference. Each of the beatings is supported by the inner circumferential portion and an annular portion, which faces inward, of each of the bushes. The rotating shaft is supported by each of the bearings so as to rotate freely. Furthermore, a sealed-structure is adopted for covering the entire stepping motor by a housing in order to prevent moisture penetrated into the bearings from the joint surface of the stator core and the opening portion of the bush.
In the stepping motor described above, the end portion of the outer circumferential portion of each of the bushes firmly contacts the entire surface of the stator core in order to prevent dust entering the bush while keeping the right position of the bearing. Since the bushes need to be formed at a high accuracy, and a sealed structure should prevent moisture to be penetrated, improvements with regard to a high production yield, decreasing of parts number and a manufacturing cost are demanded.
In a conventional stepping motor, there has been a stepping motor disclosed in Japanese Unexamined Patent Publication No. 4-372546. Here, a black anodic oxide film (alumite) is applied to a bracket (above-described conventional bush) to suppress the increase in temperature, thereby efficiently improving the rated capacity. In other examples disclosed in Japanese Examined Patent Publication No. 3-24142, Japanese Unexamined Patent Publication Nos. 1-286749 and 4-49828, and Japanese Unexamined Utility Model Publication No. 1-61854, a bracket is formed from synthetic resin, thus there are problems in accuracy, cost, and environmental durability.
For solving the above problems, there is, for example, a stepping motor disclosed in Japanese Unexamined Patent Publication No. 8-298739. This invention aims to provide a stepping motor which can be fabricated without using die-casting or cutting of an aluminum alloy by using a metal with a high yield.
In a stepping motor disclosed in Japanese Unexamined Patent Publication No. 8-298739, a stator core is disposed outside rotors which are attached to a rotating shaft, and the front portion and the back portion of the rotating shaft are held by bearings which are attached to the bushes disposed at the front and the back of the stator core, and the bush is formed of a metal plate by a press working. The portion of the bush contacting the stator core is formed so as to have a small thickness, and the portion of the outer circumference of the stator core is held by the portion having the small thickness. At the same time, the portion to which the bearing is attached has a structure in which a folding portion for holding the outer circumference of the bearing and the sticking portion for holding the outside of the bearing are arranged interchangeably.
With this arrangement, a portion of the outer circumference of the stator core is held by the portion of the bush having the small thickness formed at the portion contacting the stator core, thus a bush formed with less accuracy can firmly contact the stator core.
FIGS. 6 and 7 are sectional views of stepping motors disclosed in the above Japanese Unexamined Patent Publication No. 8-298739. In FIGS. 6 and 7, bushes 5a and 5b are formed of a metal plate by press working. The portions of the outer circumferences of the bushes, which contact a stator core 3, have a small thickness, and these portions having a small thickness (thin portions) 13c and 13d hold a portion of the outer circumference of the stator core 3. At the same time, the portions of the bushes 5a and 5b, to which bearings 7a and 7b are attached, have structures in which folding portions 21a and 21b for holding the outer circumferences of the bearings 7a and 7b and sticking portions 50a and 50b for holding the outside of the bearings 7a and 7b are arranged interchangeably.
The bushes 5a and 5b have substantially rectangular shapes of which corners are formed in a slantwise manner. The four corner portions have holes (not shown). These holes are used for leading bolts (not shown) which connect the whole structure by passing through the bushes 5a and 5b and the stator core 3.
The difference between FIG. 6 and FIG. 7 is as follows. As shown in FIG. 6, in a structure in which the stator core 3 is sandwiched between the bushes 5a and 5b, a portion of the outer circumference of the stator core 3 is held by the thin portions 13c and 13d, and the bushes 5a and 5b are tightened with bolts. In the following, the structure in which a portion of the outer circumference of the stator core is held by the bushes is called an “outer-case” type.
As shown in FIG. 7, in a structure in which the stator core 3 is sandwiched between the bushes 5a and 5b, a portion of the inner circumference of the stator core 3 is held by the thin portions 13a and 13b, and the bushes 5a and 5b are tightened with bolts. In the following, the structure in which a portion of the inner circumference of the stator core is held by the bushes is called an “inner-case” type.
In the stepping motors shown in FIGS. 6 and 7, the bushes conventionally made of aluminum alloy by die casting or by cutting can be formed by press working, and the junction portion with the stator core and attachment portion of the bearing are changed.
In the above invention, a portion of the outer circumference (FIG. 6) or the inner circumference (FIG. 7) of the stator core is held by the thin portions 13a and 13b (13c and 13d) which are formed at the portion contacting the stator core.
However, when installing the stepping motor into a vehicle, the following problem can be found. A portion of the outer circumference or inner circumference of the stator core 3 is held by the thin portions 13a and 13b (13c and 13d) formed at the portion contacting the stator core. Perpendicularly to a rotating shaft 6, the space between the stator core 3 and rotors 11a and 11b can be of a high precision by accurately forming the bushes 5a and 5b, and the stator core 3. However, in the direction along the rotating shaft 6, it is difficult to achieve a high precision since spacers 15 and 16, and the bearings 7a and 7b are interposed between the bushes 5a and 5b, so that many factors such as the forming accuracy, the assembly tolerance, and the wearing of the spacers 15 and 16, and the bearings 7a and 7b as a result of rotation are affected.
Also, the stator core 3 functions as a part of the sealed motor. However, the stator core 3 is formed by laminated silicon steel plates in general, and paint is applied on the surface of the steel plates for the prevention of corrosion, and is laminate fixed by varnish and so on. Accordingly, the surface has concavities and convexities, thus, it is difficult to prevent penetration of moisture. Further, the bushes 5a and 5b are tightened by bolts, which also makes it difficult to prevent the penetration of moisture, thus the motor is not suitable for using in an adverse environment, such as for a vehicle described above. Moreover, it requires a housing to cover the entire stepping rotor in order to prevent moisture from penetration to the bearings from the contacting faces of the stator core, and the opening portion of the bushes.